The procedure of directing a laser beam obliquely onto the surface of a glass plate in order to determine its thickness is known, for example, from European patent publication 0,248,552. The laser beam is partially reflected at the glass plate and partially refracted into the same. The refracted component is then partially reflected at the rearward surface of the glass plate. The component reflected at the rearward surface then hits the forward surface again and is refracted out of the glass. In the case of the forward and rearward surfaces being plane parallel to each other, the beam is displaced parallelly to the component beam which was reflected directly at the forward surface. The spacing between the two component beams is directly proportional to the thickness of the glass when the angle of incidence on the surface of the glass and the glass refractive index are constant.
The necessary oblique incidence of the laser beam on the surface of the measurement object is achieved in the state of the art, as a rule, by an oblique setting of the laser or by means of one or more mirrors or by means of a deflecting prism.
An arrangement of the above type is disclosed in German patent publication 4,143,186. The arrangement described in this document operates with two opposing beam paths in order to compensate measurement errors that can, for instance, occur if the measurement object is tilted. The known arrangement includes two laser light sources, two beam splitters, two linear sensors and a deflecting prism. The opposing beam paths are provided in that the optical components described above are symmetrically arranged on the deflecting prism in such a way that the symmetry axis of the prism defines the symmetry axis of the entire arrangement.
The disadvantage of this arrangement is that every measurement beam must pass through eight glass/air boundary surfaces (beam splitters, deflecting prism) on its path from the source to the detecting device, without counting the surface of the measurement object. This can affect the accuracy of measurement, since the boundary surfaces can easily be contaminated, especially when utilizing such an apparatus in manufacturing plants.
Furthermore, the two beam paths cannot be operated simultaneously, but must be operated alternately, since parasitic reflections would otherwise disturb the measurement. These parasitic reflections are caused by each of the incident beams and, after reflection in the beam splitter, can become superposed on the component beam pairs to be detected.